Preform and method of repairing nickel-base superalloys and components repaired thereby

ABSTRACT

A process for repairing a turbine component of a turbomachine, as well as a sintered preform used in the process and a high gamma-prime nickel-base superalloy component repaired thereby. The sintered preform contains a sintered mixture of powders of a cobalt-base braze alloy and a cobalt-base wear-resistant alloy. The braze alloy constitutes at least about 10 up to about 35 weight percent of the sintered preform and contains a melting point depressant such as boron. The preform is formed by mixing powders of the braze and wear-resistant alloys to form a powder mixture, and then sintering the powder mixture. To use the preform, a surface portion of the turbine component is removed to expose a subsurface portion, followed by diffusion bonding of the preform to the subsurface portion to form a wear-resistant repair material containing the braze alloy dispersed in a matrix of the wear-resistant alloy.

BACKGROUND OF THE INVENTION

The present invention generally relates to superalloy structures subjectto excessive wear, such as components of gas turbines and otherturbomachinery. More particularly, this invention relates to a method ofrepairing worn surfaces of a gas turbine bucket formed of nickel-basesuperalloys that are prone to cracking when welded.

Superalloys are used in the manufacture of components that must operateat high temperatures, such as buckets, nozzles, combustors, andtransition pieces of industrial gas turbines. During the operation ofsuch components under strenuous high temperature conditions, varioustypes of damage or deterioration can occur. For example, wear and crackstend to develop on the angel wings of latter stage buckets as a resultof rubbing contact between adjacent nozzles and buckets. Because thecost of components formed from superalloys is relatively high, it istypically more desirable to repair these components than to replacethem. For the same reason, new-make components that require repair dueto manufacturing flaws are also preferably repaired instead of beingscrapped.

Methods for repairing nickel-base superalloys have included gas tungstenarc welding (GTAW) techniques. GTAW is known as a high heat inputprocess that can produce a heat-affected zone (HAZ) in the base metaland cracking in the weld metal. A filler is typically used in GTAWrepairs, with the choice of filler material typically being a ductilefiller or a filler whose chemistry matches the base metal. An advantageof using a ductile filler is the reduced tendency for cracking. Anexample of weld repair with a ductile filler is the use of IN617 andIN625 superalloys to repair worn angel wings of buckets cast from IN738and equiaxed nickel-base superalloys such as GTD-111. A significantadvantage of using a filler whose chemistry matches the base metal isthe ability to more nearly maintain the desired properties of thesuperalloy base material. An example of this approach is weld repairingGTD-111 superalloy buckets with weld wires formed of GTD-111 or René 80superalloy. To reduce the likelihood of cracking, the base metaltypically must be preheated to a high temperature, e.g., about 700 to930° C. With either approach, the GTAW process can distort the basemetal due to the build up of high residual stresses. Components withcomplex geometries, such as buckets of gas turbines, are less tolerantof distortion, to the extent that GTAW may not be a suitable repairmethod, particularly if a ductile filler cannot be used.

More advanced directionally-solidified (DS) nickel-base superalloys areoften not as readily weldable as the GTD-111 superalloy, furtherincreasing the risk of cracking in the weld metal and within the HAZ ofthe base metal. A notable example is the nickel-base superalloy GTD-444,which is finding use for latter stage (e.g., second or third stage)buckets in advanced industrial gas turbines due to its desirable creepresistance properties. GTD-444 is not readily weldable primarily due toits higher gamma prime (y′) content (about 55 to 59%), and previousattempts to weld it have produced unacceptable cracking in the basemetal HAZ and weld metal.

In view of the above, alternative repair methods are required to repairhigh gamma-prime nickel-base superalloys that will yield crack-freerepairs. For repairing the wear-prone surfaces of such superalloys, itis also necessary that the repair material also exhibit excellent wearproperties. One such approach is termed activated diffusion healing(ADH), examples of which are disclosed in commonly-assigned U.S. Pat.Nos. 5,902,421 and 6,530,971. The ADH process employs an alloy powder ormixtures of powders that will melt at a lower temperature than thesuperalloy component to be repaired. If two powders are combined, one ofthe powders is formulated to melt at a much lower temperature than theother powder, such that upon melting a two-phase mixture is formed.Vacuum brazing causes the braze powder mixture to melt and alloytogether and with the superalloy of the component being repaired. Apost-braze diffusion heat treatment cycle is then performed to promotefurther interdiffusion, which raises the remelt temperature of the brazemixture.

Another alternative repair approach disclosed in commonly-assigned U.S.Pat. No. 6,398,103 to Hasz et al., involves brazing a wear-resistantfoil to a worn surface of a component. The foil is formed by thermalspraying a wear-resistant material on a support sheet. Suitablewear-resistant materials include chromium carbide materials andCo—Mo—Cr—Si alloys, such as the commercially-available TRIBALOY® T400and T800 alloys. Still another approach disclosed in commonly-assignedU.S. patent application Ser. No. 10/708,205 involves the use of a brazetape formed by firing a pliable sheet containing powders of a brazematerial and a wear-resistant alloy in a binder. The tape is applied tothe repair surface, after which a heat treatment is performed to causethe braze tape to diffusion bond to the repair surface so as to define abuilt-up surface, which can then be machined to the desired dimensionsfor the repair.

With the advent of more highly alloyed superalloys, improved repairmethods and materials are required that are specialized for theparticular surface being repaired, including the superalloy and thestrength and microstructure required by the repair. A notable example isthe need for materials and processes tailored to perform repairs oncomponents with complex geometries and formed of superalloys having highgamma-prime contents, such as GTD-444.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a process capable of repairing a surfaceof a turbine component of a turbomachine, as well as a sintered preformused in the process and the turbine component repaired by the process.The process and preform are particularly well suited for repairing aturbine component formed of a nickel-base superalloy having a highgamma-prime content, a particular example of which is the GTD-444superalloy. The process of the invention can be carried out attemperatures that are sufficiently low to minimize distortion, which isparticularly advantageous when repairing complex geometries, such as theangel wings of a bucket of an industrial gas turbine.

The sintered preform employed in the invention consists essentially of asintered mixture of powders of a cobalt-base braze alloy and acobalt-base wear-resistant alloy. The cobalt-base braze alloyconstitutes at least about 10 up to about 35 weight percent of thesintered preform and contains a sufficient amount of boron so that thecobalt-base braze alloy has a melting temperature of about 2000° F. upto about 2230° F. (about 1090° C. up to about 1220° C.).

A process for using the sintered preform to repair a turbine componentof a gas turbine involves preparing the sintered preform by mixingpowders of the above-noted cobalt-base braze alloy and cobalt-basewear-resistant alloy to form a powder mixture, of which at least about10 up to about 35 weight percent is the cobalt-base braze alloy, andthen sintering the powder mixture to form the sintered preform. Use ofthe preform involves removing a surface portion of the turbine componentto expose a subsurface portion of the turbine component, and thendiffusion bonding the sintered preform to the subsurface portion of theturbine component to form a wear-resistant repair material consisting ofthe cobalt-base braze alloy dispersed in a matrix of the wear-resistantcobalt-base alloy. Thereafter, machining of the repair material can theperformed to obtain desired final dimensional and surface properties.

The resulting repaired turbine component is preferably a nickel-basesuperalloy having a composition and gamma-prime content that renders theturbine component prone to cracking if subjected to gas tungsten arcwelding. Such a repaired turbine component is characterized by having aregion with the wear-resistant repair material diffusion bonded thereto,in which the wear-resistant repair material consists of the cobalt-basebraze alloy dispersed in a matrix material of the wear-resistantcobalt-base alloy.

In view of the above, it can be seen that the invention provides aprocess and material for repairing an advanced nickel-base superalloythat is prone to cracking if an attempt were made to weld repair thesuperalloy, particularly if using a filler material with propertiessimilar to the base metal. Instead of welding, the invention employs adiffusion bonding cycle that avoids the thermal stresses and distortioninduced by welding, yet yields a repaired region whose properties arecloser to that of the base metal than would be possible if a weld repairwas performed with a ductile filler material.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a casting for a third stage bucket prepared forrepair in accordance with a repair process of this invention.

FIGS. 2 and 3 represent sintered preforms suitable for repairing thebucket of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 represents a third stage turbine bucket 10 of a type used withinthe turbine section of an industrial gas turbine. The bucket 10 isrepresented as a casting prior to final machining, and includes anairfoil 12 extending from a root portion 14. Various high-temperaturematerials can be used to form the bucket 10, notable examples of whichinclude the commercially-known GTD-111 and GTD-444 nickel-basesuperalloys. The present invention is particularly concerned withcomponents formed of highly alloyed nickel-base superalloys having highgamma-prime contents, such as GTD-444, whose nominal composition, inweight percent, is about 9.5-10% chromium, about 7-8% cobalt, about3.35-3.65% titanium, about 4.1-4.3% aluminum, about 5.75-6.25% tungsten,about 1.30-1.705 molybdenum, about 4.60-5.0% tantalum, about 0.06-0.1%carbon, about 0.0080-0.010% zirconium, about 0.008-0.0105% boron, andthe balance being nickel and incidental impurities. The GTD-444 isformulated as a directionally-solidified (DS) alloy, and has a highgamma-prime content (about 55 to 59%). The high gamma-prime content of asuperalloy such as GTD-444 renders the superalloy susceptible tocracking when an attempt is made to perform a weld repair. The inventionalso concerns the repair of nickel-base superalloy components havingcomplex geometries, and are therefore susceptible to distortion if aweld repair is attempted. When formed of GTD-444, the bucket 10 depictedin FIG. 1 is an example of both complicating circumstances, particularlyin the region surrounding the angel wings 16, whose complex geometriescan be easily distorted by welding.

As known in the art, the angel wings 16 are configured for sealing withadjacent nozzle stages (not shown) of the gas turbine in which thebucket 10 is installed. Each wing 16 terminates in a tip 18 that issubjected to damage from rubbing contact with seals on the adjacentnozzles. Contact between the tips 18 and nozzle is characterized by highcompression forces and relative movement as a result of manufacturingtolerances, differing rates of thermal expansion, and dynamic effectsduring operation of the turbine. As such, the angel wings 16 and theirtips 18 are prone to damage that necessitates repair. For this purpose,the bucket 10 is shown with a surface region removed on its root portion14, exposing a subsurface region 20 that encompasses both angel wings 16on one side of the bucket 10. As such, FIG. 1 represents a first step ofa process for repairing the bucket 10, by which worn or damaged surfaceportions of the wings 16 have been removed.

FIG. 2 depicts a sintered preform 22 sized and shaped to replace thebase material that was removed to expose the subsurface region 20 inFIG. 1. A preform 24 configured for repairing only the tip 18 of anangel wing 16 is depicted in FIG. 3 (not shown to the same scale asFIGS. 1 and 2). According to the present invention, the preforms 22 and24 contain up to about 90 weight percent of a wear-resistant cobalt-basealloy, and the balance essentially a cobalt-base braze alloy containinga melting point depressant, preferably boron, to enable diffusionbonding of the preforms 22 and 24 to the bucket 10 at temperatures belowthe recrystallization temperature of the base superalloy, which is about1230° C. for GTD-444. As used herein, the term cobalt-base specifies analloy whose predominant constituent is cobalt. Preferred properties ofthe braze alloy include a melting temperature of up to about 1220° C.,compatibility with GTD-444, moderate wear properties, hardness, andoxidation resistance, machinability, and low tendency for cracking. Apreferred braze alloy is based on the commercially known superalloy MarM 509B, and has a nominal composition, by weight, of about 24% chromium,about 10.8% nickel, about 7.5% tungsten, about 4% tantalum, about 0.25%titanium, about 2.7% boron, about 0.6% carbon, the balance cobalt andincidental impurities. Suitable compositional ranges for theconstituents of the braze alloy of this invention are, by weight, about22.00 to about 24.75% chromium, about 9.0 to about 11.0% nickel, about6.5 to about 7.6% tungsten, about 3.0 to 4.0% tantalum, about 2.60 to3.16% boron, about 0.55 to about 0.65% carbon, about 0.15 to 0.30%titanium, about 0.30 to 0.60% zirconium, up to 1.3% iron, up to 0.4%silicon, up to 0.10% manganese, up to 0.015% sulfur, and the balancecobalt and incidental impurities.

Because of the presence of the braze alloy in the preforms 22 and 24,the wear-resistant alloy may have a melting temperature that exceeds themelting temperature of the braze alloy though less than the GTD-444 basemetal, e.g., above 1090° C. but less than 1315° C. Preferred propertiesof the wear-resistant alloy include compatibility with GTD-444, lowtendency for cracking, moderate wear properties, hardness, and oxidationresistance, and machinability. Two cobalt alloys based oncommercially-known hardface materials are identified with this inventionas being suitable for use as the wear-resistant alloy. A first is basedon a cobalt-base alloy commercially available from the Deloro StelliteCompany, Inc., under the name TRIBALOY® T800. The T800-type alloycontains, by weight, about 27 to about 30% molybdenum, about 16.5 toabout 18.5% chromium, about 3.0 to about 3.8% silicon, up to 1.5% iron,up to 1.5% nickel, up to 0.15% oxygen, up to 0.03% sulfur, up to 0.03%phosphorus, and up to 0.08% carbon, the balance cobalt and incidentalimpurities. A preferred composition for a T800-type wear-resistant alloyfor use in this invention is, by weight, about 29% molybdenum, about 18%chromium, about 3.5% silicon, about 0.08% carbon, and the balance cobaltand incidental impurities. The second alloy suitable for use as thewear-resistant alloy of this invention is based on a cobalt alloycommercially available from various sources under the name CM 64, anexample of which is available from the Deloro Stellite Company, Inc.,under the name STELLITE® 694. A suitable composition for a CM 64-typewear-resistant alloy is, by weight, about 26.0 to about 30.0% chromium,about 18.0 to about 21.0% tungsten, about 4.0 to about 6.0% nickel,about 0.75 to about 1.25% vanadium, about 0.7 to about 1.0% carbon, upto 3.0% iron, up to 1.0% manganese, up to 0.5% molybdenum, up to 1.0%silicon, up to 0.05% boron, and the balance cobalt and incidentalimpurities. A preferred composition for a CM 64-type alloy is, byweight, about 28% chromium, about 19.5% tungsten, about 5% nickel, about1% vanadium, about 0.85% carbon, and the balance cobalt and incidentalimpurities.

The preforms 22 and 24 are formed by mixing powders of the braze andwear-resistant alloys. Suitable particle size ranges for the braze andwear-resistant alloy powders are −325 mesh size. The braze alloy ispresent in the preforms 22 and 24 in an amount to achieve metallurgicalbonding with the wear-resistant alloy and the base metal of the bucket10 by boron diffusion. A lower limit for the braze alloy content in thepreforms 22 and 24 is about 10 weight percent in order to limit porosityto an acceptable level within the preform. In excess of about 35 weightpercent of the preforms 22 and 24, the braze alloy can undesirablyreduce the mechanical and environmental properties of the repair. In apreferred embodiment, the braze alloy content of the preform is about 15weight percent. Aside from the braze and wear-resistant alloys, no otherconstituents are required in the making of the preforms 22 and 24.

After mixing, the powders undergo sintering to yield preforms 22 and 24with good structural strength and low porosity, preferably under twovolume percent. During sintering, the powders are compressed to promotefusion and reduce porosity in the preforms 22 and 24. Based on thepreferred preform compositions containing about fifteen weight percentbraze alloy, the preforms 22 and 24 (and therefore repairs formed by thepreforms 22 and 24) have the following nominal compositions (excludingincidental impurities).

T800-type Preform CM 64-type Preform Cr 18.5% 27.6% Ni 0.92 5.5 W 0.6418.9 Ta 0.34 0.36 Ti 0.021 0.023 Mo 26.5 — Si 3.2 — Fe — 2.7 V — 0.91 B0.23 0.24 C 0.12 0.87 Co Balance Balance (about 49.5%) (about 42.8%)

In an investigation leading up to this invention,directionally-solidified buckets essentially of the type shown in FIG. 1and formed of the GTD-444 superalloy were machined byelectrical-discharge machining (EDM) to a depth of about 0.1 inch (about2.5 mm) to remove a surface region of the root section, essentially asrepresented in FIG. 1. Following EDM, the exposed regions were ground tocompletely remove the recast layer formed during EDM, and then cleanedwith acetone. The exposed regions were then subjected to grit blastingwith a nickel-chromium-iron grit commercially available under the nameNicroBlast® from Wall Colmonoy Corp. The grit blasting operation wasperformed to clean the exposed regions, create compressive stresses atthe surface to enhance brazeability, and deposit a smooth nickel coatingthat enhances the wettability of the exposed regions. The NicroBlast®powder had a particle size of −60 mesh, though smaller and largerparticle sizes could foreseeably be used.

For the investigation, two different preform formulations wereevaluated. The formulations contained either about 15 or about 10 weightpercent of the braze alloy, with the balance the above-noted T800-typeor CM 64-type wear-resistant alloy, respectively. More particularly, thebraze alloy had a nominal composition of, by weight, about 24% chromium,about 10.8% nickel, about 7.5% tungsten, about 4% tantalum, about 0.25%titanium, about 2.7% boron, about 0.6% carbon, the balance cobalt andincidental impurities. The T800-type wear-resistant alloy used had anominal composition of, by weight, about 29% molybdenum, about 18%chromium, about 3.5% silicon, about 0.08% carbon, and the balance cobaltand incidental impurities. The CM 64-type wear-resistant alloy had anominal composition of, by weight, about 28% chromium, about 20%tungsten, about 5% nickel, about 3% iron, about 1% vanadium, about 0.9%carbon, and the balance cobalt and incidental impurities.

The powders were then mixed and underwent sintering in molds to producepreforms having thicknesses of about 0.1 inch (about 2.5 mm), and aporosity of less than two volume percent. After cutting the preforms bywater jet and EDM to obtain shapes similar to that shown in FIG. 2, thepreforms were tack-welded to the exposed surface regions of the buckets.

The preforms were diffusion bonded to the exposed surface regions usingone of two vacuum heat treatments. The heat treatment for the preformscontaining the T800-type wear-resistant alloy comprised heating at arate of about 25° F./min (about 14° C./min) to a soak temperature ofabout 1200° F. (about 650° C.) held for about thirty minutes, heating ata rate of about 25° F./min to a soak temperature of about 1800° F.(about 980° C.) held for about thirty minutes, heating at a rate ofabout 35° F./min (about 20° C./min) to a maximum soak temperature ofabout 2210° F. (about 1210° C.) held for about twenty minutes, furnacecooling to a temperature of about 2050° F. (about 1120° C.) and holdingfor about sixty minutes, furnace cooling to a temperature of about 1500°F. (about 815° C.), and finally cooling to room temperature. The heattreatment cycle for the preforms containing the CM 64-typewear-resistant alloy was essentially identical except for the use of amaximum soak temperature of about 2240° F. (about 1227° C.). All repairswere machined following heat treatment to about the desired dimensionalcharacteristics.

Metallographic sections of some of the repaired angel wings showed therepairs to be very homogeneous and the entire bond interface to be voidfree, yielding an excellent metallurgical joint. Other repaired bucketswere nondestructively examined by fluorescent penetrant inspection(FPI), which evidenced that the repair and the underlying superalloybase metal were free of cracks.

In a subsequent investigation, the tips of buckets formed of GTD-444were repaired with a preform formulation containing about 15 weightpercent of the braze alloy and the balance the T800-type wear-resistantalloy. Following a braze heat treatment essentially as described abovefor the previous preform formulation containing the T800-typewear-resistant alloy, the blades were crack-free and the resultingrepairs exhibited better wear resistance than the original GTD-444material.

While the invention has been described in terms of particularembodiments, it is apparent that other forms could be adopted by oneskilled in the art. Therefore, the scope of the invention is to belimited only by the following claims.

1. A sintered preform consisting essentially of a sintered mixture ofpowders comprising a cobalt-base braze alloy and a cobalt-basewear-resistant alloy, the cobalt-base braze alloy constituting about 10to about 35 weight percent or the sintered preform and containing asufficient amount of boron so that the cobalt-base braze alloy has amelting temperature of about 1090° C. to about 1230° C., wherein: thecobalt-base braze alloy consists of, by weight, about 22.00 to about24.75% chromium, about 9.0 to about 11.0% nickel, about 6.5 to about7.6% tungsten, about 3.0 to 4.0% tantalum, about 2.60 to 3.16% boron,about 0.55 to about 0.65% carbon, about 0.15 to 0.30% titanium, about0.30 to 0.60% zirconium, up to 1.3% iron, up to 0.4% silicon, up to0.10% manganese, up to 0.015% sulfur, and the balance cobalt andincidental impurities; the cobalt-base wear-resistant alloy consists of,by weight, about 26.0 to about 30.0% chromium, about 18.0 to about 21.0%tungsten, about 4.0 to about 6.0% nickel, about 0.75 to about 1.25%vanadium, about 0.7 to about 1.0% carbon, up to 3.0% iron, up to 1.0%manganese, up to 0.5% molybdenum, up to 1.0% silicon, up to 0.05% boron,and the balance cobalt and incidental impurities; and the sinteredpreform has a porosity of less than 2 volume percent as a result ofbeing subjected to compression to promote fusion and reduce porosityduring sintering.
 2. The sintered preform according to claim 1, whereinthe sintered preform is attached to a gas turbine engine componentformed of a nickel-base superalloy having a gamma-prime content of about55 to about 59 volume percent.
 3. The sintered preform according toclaim 2, wherein the nickel-base superalloy consists of, by weight,about 9.5 to about 10 chromium, about 7 to about 8 cobalt, about 3.35 toabout 3.65 titanium, about 4.1 to about 4.3 aluminum, about 5.75 toabout 6.25 tungsten, about 1.30 to about 1.70 molybdenum, about 4.60 toabout 5.0 tantalum, about 0.06 to about 0.1 carbon, about 0.0080 toabout 0.010 zirconium, about 0.008 to about 0.0105 boron, and thebalance being nickel and incidental impurities.
 4. The sintered preformaccording to claim 1, wherein the cobalt-base braze alloy consists of,by weight, about 24% chromium, about 10.8% nickel, about 7.5% tungsten,about 4.0% tantalum, about 2.7% boron, about 0.6% carbon, about 0.25%titanium, and the balance cobalt and incidental impurities.
 5. Thesintered preform according to claim 1, wherein the cobalt-basewear-resistant alloy consists of, by weight, about 28% chromium, about19.5% tungsten, about 5% nickel, about 1% vanadium, about 0.85% carbon,and the balance cobalt and incidental impurities.